Estimate of Asteroid Brightness in the Mid-Infrared

Estimate of Asteroid Brightness in the Mid-Infrared

Radius (km):

Distance (A.U.):

Wavelength (microns):

Filter name?

To obtain a rough estimate of the brightness of an asteroid of a given
radius at a given distance please enter values in the form above. You can
either get a monochromatic flux value or the in-band flux value in one of
the Michelle filters.

These values are calculated from a simple blackbody fit to the ISO spectrum of
asteroid Ceres. To scale these values one must assume that all asteroids have the
same surface characteristics as Ceres, and that the surface temperature is not a function
of distance from the Sun. Of course neither of these is true but this will still give a
rough estimate of the brightness. In one case where an asteroid was acquired for
spectroscopy the images implied a flux density that was 0.85 times the value one obtains
from this page, which gives an idea of the uncertainties involved in using the values
given here. One should probably assume 15% to 30% uncertainties.

This form is intended to be used to get an initial idea of whether a specific asteroid
can be used as a calibration target for high resolution spectroscopy with Michelle,
given the distance at a specific time, the estimated asteroid radius, and the wavelength of
interest. The asteroid has to be fairly bright (say 40 Jy or more, preferably more)
so that its spectrum gets good signal-to-noise ratio in a relatively short time.

The values given by this web form are uncertain because the ISO spectrum of Ceres
appear to be subject to slit losses of about a factor of 2. It is possible that the
slit losses have been underestimated. The normalization was determined from a Michelle
spectral observation of Vesta, and the slit used for these Vesta observations was wide
enough that slit losses should have been small (a 0.8 arc-second slit was used,
compared to the Vesta image FWHM of 0.45 arc-seconds and the stellar FWHM of 0.35
arc-seconds at the time).

The filter brightnesses are provided in case one wants to take an image of a large
asteroid with Michelle. If the asteroid is too bright it will saturate. Ceres near
opposition is too bright to image with Michelle in the N-band filters.

A plot of the Ceres ISO data and the fit is shown below.

I used a Michelle observation of Vesta to set the absolute level of the simulated fluxes.
Vesta was observed spectroscopically and photometrically on 2005 December 18 (UT date) when
Vesta was at a distance of 1.612 astronomical units. The imaging observations are not
saturated and the low resolution spectrum appears to be well calibrated, so those observations
determine the absolute scaling of the values given on this page. The Ceres ISO observations
were taken at different times and scaling factors were applied to produce a smooth curve
for the fitting, as indicated in the plot. While Ceres was at a distance of 2.3
astronomical units when some of the ISO SWS observation were taken, scaling from
this distance and the radius of Ceres results in values that are a factor of 2 lower
than those determined from the Vesta observation. It seems unlikely that this discrepancy is
due to slit losses in the ISO spectra, or to differences in emissivity in the mid-infrared for
Vesta and Ceres. Another possibility is a difference in illumination and limb darkening
between the two observations, since Vesta was observed near opposition while the ISO
observations of Ceres were taken at a solar angle of more like 90 degrees. However I am
dubious that the effect could be this large.

Due to this unresolved discrepancy values from this page are rough estimates only, and
in choosing asteroid standards people should try to make sure that the S/N estimated from the
Michelle ITC for the asteroid is larger than 100 in of order 30 to 60 seconds on-source time.